Frequency meter



Aug. 29, 1950 J. EWERNER ETAL 2,520,867

FREQUENCY METER Filed Feb. 21, 194s 2 sheets-sheet 1 INVENToRs. Jaim E Wenzel: Haroldechtol and Edy' W BIeiSCI BY g 5 THEIR A T T ORNE Y Aug. 29, 1950 J. E. WERNER ET AL.

FREQUENCY METER Filed Feb. 2l, 1948 2 Sheets-Sheet 2 '72 A 25 B71 N52 @Ll 27 20 F X5 121:5 H124 28 TL 57 35?-105 45' w1? 51/ m X4 X5 x? X1 WL a E n m .LBJ

We @im v/ OSI 0.51? 05'5 054 IHF Fig, 5.

and Bfeiych THEIR ATTORNEY .appear as. distortion atenteclug. 2.9, 1950 2.529,18@ FREQUENCYMETE..

John-EiZWernenrGlenshaw,-andliarold'll-:Boohtnh andEdgarW.. .Breisoh Edgewood, Pa assign. s..

. Sii/itch. and Signal.. Campania.' [a *oorporatiiili. .0 PsnnSylY/fania Application-February 21, IMS-Serial.Noitlll 4 tCliiiS... (01.,2501-39 A).

rangeof communoatiom ae. Meli. as-.anfinoreasesi tendency.. tofnroduoeean. undesirable.- amplitude modulation. may.. result frein` a comparatively slight dritt the...I transmitter osoillator free enemy; Attnef. reeeivingz.A station... improper maintenance otftiietransmitter frequency. will speech. Inaccuraoy 1,v Y l if extreme; may.. cause@oyerloadinein99W@ ampiier tubes.. Ordin-amm proio1ed...with trant? mitters of Atheftype. nere-involved; ,wit quent shorteningiottneelife 'theetu .es multiple@channehsystems U .l str-anse mitter eapableetpbeingedierentiyeoonditionesii@ supply. theoifferentenannel .freqneneies it in possible to ...maintain n openbalanoeebetween .the differente onannel .f load erre on. t1ie,,=nower amplier: ofthe ltransmit,ter-.1i th fre: queries' offtne :difierentohannels-.isf not-:keptfas close as .possibletotheir-respective .valuese These eondtions arising in FM-oommunoation systems vmaite.; it` essential.. that. thefirequenoy; of the transmitters-.of :suon-.systems.fbercheokedtpee riodeallrandin aotialmosta :dailyeoheekeisdee sirable.

In railway traint-.communication systems, .the inspecting .and testing;of,v .the trainfarreg. onnipment is done at engine houseszorrterminal. points while the..locomotiveL or. other vehicle..on whieh theequipmentisfmountemis-being madeiready for-I aftrp.- That .,is;to 4.-s.ay, tneaY testing g andt, .in. specting of.. siichA trein.. carrieoluft equipment are done. While otherservicingeof the Y.veia'oleis going .on and are often.- done=..under adverse. conditions ystemsfuse frequency.

y oommiinioation .grounet in a.- single: r-eeentable oiftheplug-f.

une .offpowerfanitpreiere ably Without the neoessitlygotaroltage.eomeetitlg device..

Eurthermoref.;afrailxvay.; have .one train .2"1 ton-use z .an ...available-1f s temforemameline t .sefaifV artis a d ter using@ diierelit ehanrieL-f able therefore-ir the-.testine other, systems ,I difelent o syste enemies .Itis Qithef; trali.. systemsof a ra meten.

ing-.,ftnansmitterse.otommiineationsy me Another feature of our invention .irrthepgoe vision. of-faenovefaneeusefut freeuemneheginng meter whichfis ortablerueeed and embodies tno1-.anhalten.eteristiV y; oheokin, Y, meter which iseapable-ot .usinefthefpotversource-of .the transmitter being tested angl.,without` thefusef .speeiaivoltage-.eenye et/ieee... y

Anothenfeatur ou lriverrtionzis theiprot'ie sion ola-frente Y.- hee1ine,.,-meter-capable of beineusedA for.. testinaV t.th ....Aansrnitters .ofseveral different channel frequencies.

Qther features, obieetstand advantages-,of .our invention will .anpearfase tne..-.speoiiioation noe grosses.-

. To attain the foreeonegfeaturesg;obieotsand advantagesoitourinve tion rotffdefaf-meter haring.;v Comparison. meansi; anni un audible frequeney comparison Thesedifferent .elements are arraneedas a oo assembly; or untadapte .able .ot being-hou .relativelysmallportable ease. This .oaseeis..-:provitee.gwith 'a :ton panel on which there.; are .grouped .thefadjustnggcontrol tif, the.;arrangement-.bein such v.that .theflevers e, indcatorsiare .reanilyaeoeesblo forioperatioa. d the.. indieators .easy topbserve. AiSo-fthe .Heee-.Se sary input and power terminals of the metenere We and `iivhien reeeptaeleojis. mounted.. on. the parte! so 1that .tnemiete'rcan ,leV euielt-lygandieasily. cone l netted to the transmitter to .be-.-.tested, sueneen. neetions beingeeeteelfb single trahie provided. at one end with.,azfplugiypereonneetortzthe 111 ...and

a.osoillationt:neon .a.,visual..irequeney Y 'n in Fig; 1.

receptacle and at the other end with clips or plugs for connection to the different terminals oi the transmitter. rihe meter is provided with a head receiver for use with the audible frequency comp parison means, this head receiver being fitted with a cable of convenient length and which cableY is formed at one end with a plug contact for insertion into a jack located on the meter panel. The case of the meter is preferably provided with a compartment in which the power and input cable as well as the head receiver and its cable can be stored when the meterl is not in use.

l/Ve shalldescribe one 'form of meter embodying our invention and shall then point out the novel features thereof in claims.

In the accompanying drawings, Fig. 1Y is a trop plan view with the cover removed showing a preferred form of meter embodying our-invention when used with a railway train communication system. Fig. 2 is a side sectional view of the meter of Fig. 1. Fig. 3 is a diagrammatic'view showing the apparatus and circuitsoffthe meter of Figs. 1 and 2.

It is to be understood that our meter" is not t portion if! by hinges I2 and latches I3, the hinges I2 being preferably of the slip joint type to permit the Vcover tov be readily removed for more convenient Vuse of the meter." VA handle I4 is secured fto the-case CA to-facilitate `the carrying ofthemeter. f Y l I #The'box portion I 'of the case CA'is formed with a'large compartment Iin which the apparatus and vcircuits to vbe referred to hereinafter are-l housed, and with av small compartment I6 irrj'whiclfi'V --the 'connection cable and the head receiver provided for lthe -nieterl can be stored When-the meteris not in use. g y Y I `In practicing the invention we have vfound that af wooden case of approximately-- inches long, 1,4 inches wide and 7 inches high, 'formsaJ suitableV housing forY the meter embodying our invention. f

A top panel il of suitable material is ntted over the compartmentv I5of the case and` this top panel has mounted thereon the several manrial'ly operable control levers ofthe meten-a visualy indicating screen, 4:it-'jack'for'the head receiver anda receptacle for terminal connections, and all'f-o'f which elements will; be referred to from tiineto timeas the specification progresses. As best shown in Fig.v 2, a subpanel l! is supported beneath the panel I7 i'n the compartment I5'a's b'ywlong screws or other fasteners and the two panels Il and e form a frame on which the different elements of the meter are mounted -in a compact assembly sor unit, 'and which assembly is vsecured in place vin Yther compartmentY I5 as by screws shown in the extreme corners of the panel i' "Referring to FigSfan oscillation means of the meter includes anl electron-tube 2Q, a. group of four crystals XL-XZ, X3,"and X4, a corresponding group of tour oscillatory circuits OSI, OS2, 0573,and'OS4, and a sele'ctorswitch SS together witlrsuitable circuit connections.

The selector switch SS comprises contact members 2l and 22 operatively connected together as indicated by a dotted line 23, and further operatively connected by a shaftlike member, not shown, to a manually operable control lever 25 mounted on the top of the meter panel II. The control lever 24 is operable to four different positions which are designated by numerals 1, 2, 3, and 4 as will be readily understood by an inspection of Fig. l. Thus the two contact members 2I and l22 of the selector switch are moved in a stepf-'by-step manner bythelopelration of the control lever 24; thecontadt members 2l and 22 each having four distinctive contact positions. The contact member 2I selects by its different positions the crystal and the contact member 22 selects by its different positions, the oscillatory fcircuit to be connected to the tube 2U. The crystals XI to Kil ma-yibe of any one of several iorms `of crystalsand each is ground to be resonant at a given individual frequency. Each of the oscil- V'latory circuits comprises an inductive coil and 32,533, 31S, and 35,-l and 'disposed-as shown in Fig.

1 to the right and to lever 22j y I As an aid in` the-understanding ofthe meter, we" shall assume by way ofillustra'tion that the crystal AXl and its corresponding circuit OSi are resonant ata carrier frequency'or" 198 kc., crystal X2 'and its corresponding .circuit OS2 are resonant at a carrier frequency of 144 kc., crystal X3` and its circuit OSS are vresonant at 103 kc., and crystal Xt and its circuit OSl areY resonant the left of theselector switch at ltc.V `It' is to be understoodV that other car-A the selector switch .SS so that a deiinite frequencyV for the oscillations created by the tube is'prcvided for each position ofthe selector switch. The tuber '-2i is-preferably an indirectly heated triode having an anode 25, a cathode 21, a control grid 21 and a' heater element '28. The heater 28 is provided-*with a circuit connection extending from ya terminal'N32l of areceptacle 29 to be referred to hereinafter, through resistorsR'I 3 and R I li, heater 28 of tube 2B, a heater-320i another tubeV 3l to be described'shortly, and ground to a ground terminal ,X322= of the receptacle 2.3.The parts of this circuit Vare'so proportioned thatv the tube 2i) is properly heated' whenV a direct current voltage source of a relatively low voltage of say 32 volts is connected across the two terminals N 32 and X32 ofthe receptacle 29, such currentsourceA being at times connected Vto these terminals in a manner to f be explained hereinafter. The anode 25 of the tube 20 is powered 'by' a circuit including a terminal BTI of the receptacle 29, Yresistors'Rl and RI, a particular one of'the'oscillatcryV circuits jaccording to the position of the selector switch SYS, contact member 22, wire 63, anode 25 and tube space'to cathode 26, resistor R?, and capacitor Ctl in multiple,` ground and `terminal X32, 'Ilhe parts of this circuit are so proportioned that the tube 2t is satisfactorily powered when a direct current source of relatively high'voltage of say 350 volts, is applied between terminals BTi andiXSZ bof. the. receptacle. The control'grid '21. of tube2 issconnect'edto yground through two parallelzpaths,A one of .which includesa capacitor C6 and aparticular oneof: the crystals. accordingto the :position of the contact member 2I of .the vselectorfswitch,` and theother-sof which paths inclufdes;aresistorlt.`v It isapparentfro-m the.fore; goingvdescription of the. circuits described for.V the tube -2 that oscillations corresponding Ato the res-r onant .frequency of the particular. crystal and osa cillatory` circuit connecteditotheftube 'are pro-.- duced when-pcwerisapplled to the terminals BTI X32', and .N324 of .the receptaclel. We 'have fdund: that the. crystalcontrolled oscillator of the type hereY provided can.y be readily made to haveV a frequency drift tolerance 01?...02per cent from:0' to450." centrigrade and it will befof 'a high orderv oil-'stability underV all conditions of: operation.' l The Visual.frequencycomparison means of the meter includes a cathode ray .oscilloscope-and a controlcircuit netvvork'to supply thenecessary voltagesto thecathode' ray'tube. The cathode ray -tube13If oitheoscilloscope is of a suitable frm'and is'hel'd-ina socket carried on a 'mount-a ing bracket 5I secured to subpanelfloi the meter. The. tube is positioned for-its image to be focused by-famirror 51 on -a screen Edmountsd in the top panel Il 'of the meter. The tube 3l is provided withspecia'lattachments, not shown, that cooper-` atev withf mating devices Acarried on the panels 9 and l1 soH that the tube 'is heldiirmly inplace andv isi-cushioned Yto withstand Vibrations andy shocks lnotwithstanding the fmeter may be subjectedlto rough. handling. Therheater element 3IJ-1'oftube BI in series with the heater element 28 of-'ftubeZil vis connected across the terminals X32 and N 32 of the receptacle as previously traced. and the. tube 3iv is lheated whenever the proper voltage -is applied tothe receptacle terminals.

Horizontal deection .plates 37 of tube. 3| are coupled to the output ofithe oscillator tube 2t through a capacitor C1, and vertical deflection plates AZAlwof the tube3 Ilare connected to an input terminal CF of the receptacle 2t through anatte'nuator consisting .of a capacitor CIS, capacitor.

CISIbeing adjustableand operatively connected tofafcontrol'lever. Il mountedon the panel Il of the meter. Accordingly,z an image will be pref sen-tedon the screen 36 Iwhenever ,carrier frequency waves are applied to the two plates. 3l and 391 and the strength of the wave supplied from terminal CF to the deflection plate 39 can be manually controlled by the setting of the lever Ml.

For lpositioning .the image. Hon the. screen the horizontal and vertical delectionplates 3l and 3Q are-provided with control voltages. derived from a voltage. divider networkcomprising resistors R?, R8.. and R3,..the resistors R1. randRS in multiple being connected. in seriesvvith resistor R9 across the high -voltage terminals B'I-I and .XE2 of the receptacle. Thevoltage applied to the vertical deection plate 39 is derived from an adjustable terminal il of resistor R8 and the voltage applied to.. .the horizontal, deflection plate 31 is derived from an yadjustable terminal 42 of resistor R7. Therterminals il and 42 are operativelyconnected-:to control levers `IIB and 44, respectively,

mountedon the top rof the panel I? whereby the ligure presented on the screen can be moved to a centerposition or to a position for` best presentation byv adjusting the levers 43 and 44,

An anode 45 of the tube 3l is provided with a` voltage v:from-an adjustable terminald ofthe resistorpR Aof-the voltage divider network, and

which terminal, 6 is operatively l.connected to. a.. i

control-.lever .4 T mounted 'onthepanel' I 1 with th result the'goutlineof the gurepresentedonlthe screencan be lsharpened by adjusting thetflever II'L?.A Again,x a-'control electrode 48'of the tubeBtis providedw-ith afbias'voltage from a voltage divider comprising resistorv R6` :connected .between termi-v nals rrl\I32and.X32 yof thereceptacle 29, anfade justablel terminalAS. of resistor'l beingV connected tothe control; grid 48 andithevterminaldebeing operatively connectedA to a controlsV lever f 553 mounted on the panel Ill. Consequentlygth'ein: tensit'ytofithe .image presented on the: screen T36 can be: governedbyfadjusting. the 'control lever 5I);

Theismanner oieoperation lof this visualx-fre-L quencyLcompa-risonlmeans will be explained'more fully lwhen the. operation of. themeter is described? The-audible frequency comparison meansxgorthe. ',.meterrincludesl an",r amplifier y tube: 38 and@ ai mixer rectierf52.. Thetube 33 is preferahlyfan indirectly 1. heated .triodev similar totube i 20,1and

the. heateriielement 53 .of tube.` 38.1is connected 1in multiple 'with 'I the` .heater v element. 28 `of l tubeff2 as=..swill'..beareadilyunderstood by: an' inspectioni offliig;A 3.? Thus-the :tube .3B is 'heated whenever proper voltage is applied'to the terminals'XSZ?andl N32ofithe receptacle 29;: An anode 54 oftube Mis-powered from terminal BTI of the receptacle. throughfa'resistor RIZ vand axcontrol grid 5510i? the.tube=38:=is coupledto .the oscillator tubeZG through-capacitors C I 4 and C1;

Themixer '.rectier 52 is preferably of' the, crystalvmixer recter type but other forms-may ber-used... The input side of the mixer rectier: 52fis coupled to the input.: terminal vCF ofathe; receptaclegl; through a resistorrRIS and iscou-. pled toizthe anode `circuitv of 'the'. arnpliiierzs through .af 4capacitor, z C i5.; The Voutput ot the. mixer; rectie'r. elementv 52,r is connected ,to ground and:across the. rectiiier 52 `are connected theter. minals oa jackie mountedinv the' panell l1, the jack 5.56r being adapted to receive theplug ofthe'. cord iofv .therhead :receiver providedfor the meter:` asexplained hereinbefore.

Itlfollows from the foregoing description that: the-frequency of the oscillator tube 2G and the frequencyof energy appliedto terminal CF 'arel mixed a at; the .rectier .52 and a wave having a frequency corresponding to the heat frequencyfofi,

the two frequencies is .applied across the terminals; Offthe'iaCk Se anda corresponding: note. will .be sounded-in the head'receiver.

The receptacle -29 provided on the panel -I'I of:

themeter is'ofthe plug-in typefadapted to receive thei-,plugconnection of` a; four conductor .cable provided iorfthemeter as explained hereinbefore..v

type of plug to mate with a cooperating jacke.

The Vclips of the cable will be clearly identified to aid the operator in. making the proper power'v connections when using the meter, wellas the head receiver and its cable can be stored in the compartment I6, vas explained here: nloefore, when the meter is not inuse.

It is to bepointed out that the transmittersr tobe tested areprovided eachwith a power sourcev or power unit, .and Whichlpower unit is capable of supplying afrelatively low voltage and a rela-4v tively high voltage,V the low-*Voltage vrsource being usually oftheorder of 32 volts and the high volti-.

This cable asv age source being usually of the orderA of 35.0.Vo1ts. It is to .be understood, however, that. power sources of other voltages can be. used. In 'such power sources the positiveterminal ofthe low voltage source and the negative terminal ,of the high voltage source are connected to a common ground terminal which is here identified as terminal X32. The positive vterminal of the high voltage source is identified as BTI and :the negative terminal of the low voltage source identiedasN32." f

11n' testing the transmitter, .the meter'liC` is placed-alongside'the'equipinent box'in which the transmitter and its power vunitand usually the receiver of the communicationsystem are housed. The BTI,` X32, and N32 conductors of the meter cable are clipped onto Vthe 'corresponding terminals of the equipment box and the plug end'of the `cable is'inserted into the receptacle'29 of the meter. Also, the plug terminal of the fourth Vconductor of the cable .is inserted into ajack usually provided for the transmitterand which mitter is of the two channel type capable of supplying diierent channel frequencies, a particular one is selected, that is, the transmitter is conditioned to supply a given one of its carriers. The selector switch SS of the meter is set for its oscillator to supply the corresponding carrier frequency at which the transmitter is conditioned.

` With the wave from the meter oscillator and the Wave from the transmitter under test applied to the cathode ray tube, a resultant gure is presented on the screen 36.. The attenuator lever 40 of the meter is adjusted to obtain an approxi-V mately square figure on the screen 36. If neces sary' focus and intensity of the presentation'on lthe screen rare adjusted by the control levers 47 and 50 so as to give a clear Vpresentation-A of sufficient brilliance. Also, if the presentation is not centered properly on the screen the control levers 43 and 44 are adjusted With the head receiver adjusted to the'operator,A its cord is plugged into the jack 5f of the meter. If there is a Vdifference of the order of 30 cycles per second or more in the frequency of the oscillator of the meter and the oscillator of the transmitter an audiblev note that has a 'pitch corre-4 sponding to the difference between the two frequencies will .be heard in the head receiver.

Assuming a note is heard in the head receiver indicating that the transmitter is out of adjustment, the adjustment means provided' for the transmitter is unlocked and adjusted in theusual manner while the operator is listening to the note in the head receiver, and also at the same 8 the .voltage applied to the twofsets of deiie'ctior plates 37 yand 39'i one from' the crystal .oscillator of the ymeter and one `from the transmitter oscilf later are ofthe same'frequency a circle will .appear .on the screen 36. l As the difference between the transmitter oscillator frequency and .fthe meter crystal oscillator frequency is reduced, the presentation on the screen 36 will first shift from a circle to a Vstraightline inclined about 45. to the horizontal and back to the circle'agan.v The rate of thisk shift will decrease as the two fre-V quencies .are brought: closer together untilall motion stops, indicating that the transmitter oseeillator .has'practically thev samel frequency 'as the crystal oscillator of thermeter. At'this point the operator locks the vtransmitter adjustmentin place, the frequency of thel transmitter being again checked to assure thaty the ftransmitter frequency has not been changed in the locking of the adjustmentY means.V In'mostcases it would not be possible t0 maintainthe circle absolutely motionless but approximate'complete vsynchronism 'of the transmitter oscillations andthe oscillationsr of the crystal oscillator of the meter can be obtained.

If the original Ydifference Yin the frequency-is large, say 260 cycles per second or more, the `iig-f ure Yon the screen will appear to the eye to be the same regardless of the direction the operator adjusts the transmitter. This is because they human eye is unable to follow the circle and its changes at a rate of much-more than 25 nto' 3Q times per second. However, rthe pitch of the note heard in the head receiver is easy to observe Yand. the operator can tell quickly if the dierence between the oscillations is being increased 0r' decreased by the adjustment. vThus by the use of the audible frequency comparison means and then the visual frequency comparison means. of the meter the transmitter under test can readily and quickly be brought to its specific carrier frequency;

To check the other carrier frequency of the transmitter the procedure is substantially the same, the transmitter being conditioned to supply this vother carrier frequency and the selector switch SS on the meter being 4set for Athe corre spendingY carrier frequency.

It is apparent that by the several. adjustments of the. meter, proper intensity, focus and posi-l tion of the presentation on the screen can be quickly obtained for each of the different carrier frequencies being tested. The meter here disclosed provides four crystal oscillators for testing four different transmitterA frequencies, but it is apparent that if additional transmitter frequencies are used the meter can be equipped with additional crystal controlled circuits and the selector switch provided with additional positions. A meter of the type here disclosed has the ad-` vantages that it is portable and can easily be time observing the presentation on the screen 36.

.In case the note heard in the head receiver increases Ain pitch'the direction of movement of the transmitter adjustment should `be reversed. lf the note decreases in pitch the operator continues the adjustment until the note is no longer heard, indicating that the difference between the crystal oscillator frequency and the Vtransmitter frequency is less than. an audible frequency.

` After the raudiblenote inthe Vhead receiver is no longerheard, the adjustment is continued by observing the presentation on the Screenl.- When used at any location and testing canbe carried onV under adverse circumstances. A transmitter can be checked as to its frequency 'quickly and.' with a high degree of accuracy. Also `the meter` requires no self-contained source of power since`v meter.

Although we have hereinshown and described but one' form of meter for testing transmitters` of communication systems embodying our inven-Y 9' tion, it is understood'fthatvari'ous changes and modifications mayHbe--madextherein within the scope of the appended claims without departing from the spirit and scope of ourinvention,

Having thus described our invention, what we claim is: l

1. In a -meter for testing transmitters of'a communication systeinusing a'given carrier frequency and each of lwhich transmitters includes an oscillator supplying said Ycarrier frequency and is provided with a` high and a lowvoltage power source, the combination comprising, a standard oscillator proportioned to create oscillations of a selected carrier frequency, a visual frequency comparison means including a cathode ray tube and a screen, an audible frequency comparison means including a mixer rectifier and a head receiver, a receptacle having an input terminal and high and low voltage power terminals, said input terminal adapted to receive a carrier frequency current, said high and low voltage terminals arranged for connection to a high and a low voltage power source respectively, circuit means including manually operable control devices to connect said cathode ray tube to said standard oscillator, said input terminal and said high and low voltage terminals; said control devices operable to Condition said cathode ray tube for the figure presented on said screen to be characterized according to the difference in the frequency received at said input terminal and the frequency of said standard oscillator, and other circuit means to connect said mixer rectifier to said input terminal and said standard oscillator to create in sai-d head receiver a note the p-itch of which is determined by the difference between the carrier frequency received at said input terminal and the frequency of said standard cillator.

2. In a meter for testing transmitters of a communication system using a given carrier frequency and each of which transmitters includes an oscillator supplying said carrier frequency and is provided with a high and a low voltage power source, the combination comprising, a crystal controlled electron tube oscillator tuned to create oscillations of a given carrierfrequency; a visual frequency comparison means including a cathode ray tube, a screen and a control cincuit network; an audible frequency comparison means including an amplifier tube and a mixer rectifier adaptable of having a head receiver connected thereto, said meter having an input terminal and high and low voltage power terminals adapted to receive a carrier frequency current and a high and a low voltage power current respectively, means to connect said crystal controlled oscillator to said high and low voltage power terminals for effective energization of that oscillator, said circuit network yconnecting said catho-de ray tube to said crystal controlled oscillator and to said input and high and low voltage terminals and having manually operable control devices interposed therein to condition the oath-- ode ray tube for a given presentation on said screen when the carrier frequency received at said input terminal is equal to the frequency of said crystal controlled oscillator, and other circuit means to connect said amplifier tube and said mixer rectifier to said crystal controlled oscillator and to said input and high and low voltage terminals to create in said head receiver a note the pitch of which is determined by the difference between the carrier frequency received at said input terminal and the'fr'equency of said crystal controlled oscillator. A'3. -In a meter for testing transmitters of a communication system lusing a given carrier frequenn cy and each of which transmitters includes an' oscillator supplyingl s aid, carrier frequency and is provided with a-highand a low voltage power source, the combination comprising; a receptacle havingv an input, ahigh and a low voltage terminaly adapted-*to receive a carrier frequency current anda high-andalowvoltage power current respectively; a crystal controlled electron tube oscillator tuned to create oscillations of a selected carrier frequency and having connections to said high and low voltage terminals for energization of that oscillator, a cathode ray tube having a screen, a control circuit network including manually operable control devices to connect said cathode ray tube to said input, high and low voltage terminals and to said crystal controlled oscillator to provide on said screen a given visual presentation when the frequency of said carrier received at said input terminal is substantially equal to the frequency of said crystal controlled oscillator, an amplifier tube, a mixer rectilier, means to connect said amplifier tube to Said high and low voltage terminals and to said crytal controlled oscillator, means to couple said rectifier to said input terminal and to said ampliler tube, and a jack having connections to said rectifier and adapted to receive a cord plug of a head receiver to provide a note the pitch of which is an audible indication of the difference in the frequency of said carrier received at said input terminal and the frequency of said crystal controlled oscillator.

4. In a meter for testing the transmitters of communication systems using frequency modulation with each system using a center carrier frequency individual to the system, the combination comprising, an oscillator electron tube, a plurality of crystals each resonant at a selected individual carrier frequency, a plurality of oscillatory circuits one for each said crystal and each circuit tuned to resonance at the carrier frequency of the respective crystal, a selector switch having la. plurality of positions one for each of said crystals and the respective oscillatory circuit, a pair of power terminals adapted to be supplied with power, a first circuit means controlled by said selector switch to connect said power terminals, crystals and oscillatory circuits to said oscillator tube in such a manner as to generate oscillations of a carrier frequency selected by the position of said switch, an input terminal adapted to receive a carrier frequency current; an audible frequency comparison means including an amplifier tube, a mixer rectifier and a head phone jack having connections to the rectifier; a second circuit means to connect said power and input terminals and the oscillator tube to said amplifier tube and said rectifier in such a manner as to supply to said jack a beat frequency equal to the difference between the carrier frequency received at said input 'terminal and the carrier frequency supplied by said oscillator tube; a visual frequency comparison means including a cathode ray tube having a vertical and a horizontal deflecting plate, an anode, a cathode, a control grid and a screen; means to connect one of said deiiecting plates to said oscillator anode and the other to said input terminal, an adjustable capacitor being included in the connection to the input terminal, and a third circuit means ineluding a resistor network having adjustable terminals to connect said power terminals to saidgariode, cathode, control grid and de- Vfleeting plates of said cathode ray tube in such 12 REFERENCES CITED The following vreferences are of record in the file of this patent:

Number UNITED STATES VPATENTS Name Date Downey Oct. 15, 1935 Summerhayes, Jr. Jan. 26, 1943 Speaker Jan. 29, 1946 Walsh et al. Feb. 12, 1946 Adams Sept. 2, 194'1 

